German Patent No. DE 199 52 836 describes a method where the mass flow rate of secondary air is arithmetically determined as a function of the ratio of the intake-air flow rate to the fuel flow rate of the internal combustion engine, and as a function of the signal provided by a lambda sensor and the signal provided by an air sensor for the intake air. The evaluation of the secondary-air system is subsequently carried out as a function of the calculated mass flow rate of secondary air.
The known method assumes that the signal of the lambda sensor is available. Therefore, the known method cannot be used to carry out a diagnosis in the starting phase of the internal combustion engine, while the lambda sensor heats up to its operating temperature.
An emission control system situated in the exhaust system of an internal combustion action, e.g. a catalytic converter, generally requires a minimum operating temperature, which can be reached during the heating phase of the internal combustion engine, using a heating measure. A possible heating measure provides for combustible exhaust-gas components and secondary air to be introduced into the exhaust-gas region upstream from the emission control system, the combustible exhaust-gas components and secondary air exothermically reacting with each other and heating the exhaust gas.
An object of the present invention is to provide a method for diagnosing a secondary-air system of an internal combustion engine, which is independent of the signal of a lambda sensor.